This invention relates to pulse width modulated (PWM) control signals. More particularly, it relates to a method for producing such signals which signals may be used for a power conditioning load requiring variable amplitude and variable frequency control signals, as for instance, a DC to AC converter or power inverter used to supply power to a variable speed AC drive.
Control input signals to power inverters are typically in the form of PWM switching or control signals selected to ensure that AC power supplied to the load of the power inverter output approximates a reference analog signal, typically a sine wave. PWM inverters approximate sine-wave output waveforms by switching the power elements at a rate higher than the frequency of the reference signal. There must be a sufficient number of pulses to prevent an increase of lower order harmonics and undesirable fluctuations in the power inverter output current. The pulses must also be precisely positioned along the time axis. If there are variations in this positioning, the harmonic content of the overall PWM signal will change, causing additional power losses associated with a higher harmonic content.
A present method of modulating a reference signal employs a bipolar triangular timing wave which has two different polarities during each of its periods. The bipolar PWM control signal generated by this method contains both a positive and a negative portion during each period thereof, and thus requires that the output phase terminal, coupled to the power inverter DC supply through the switches of the power inverter, be instantaneously switched from the positive voltage of the inverter DC bus to the negative voltage of the inverter DC bus and vice versa, at the zero crossing of the control signal. However, there are physical limitations to the response time of any switching element, as for example bipolar transistors or thyristors, used in an inverter circuit. These switching elements require a finite time from the receipt of a turn-off command in which to dissipate base charge and thus to stop conducting, while they will start conducting in a much shorter time from the receipt of a turn-on command. Thus it is possible to have both switches in a leg or phase of the power inverter conducting simultaneously, which will create a short circuit or shoot-through across the DC voltage buses of the inverter. In order to avoid this shoot-through condition, it is necessary to provide a lock-out time interval during which both switches in a leg or phase of the inverter bridge are rendered nonconductive before the transition from conducting to non-conducting mode or vice versa is permitted. These lockout intervals not only complicate the control circuitry, but also add additional undesirable harmonics to the output voltage of the inverter, which causes additional power losses associated with the higher harmonic content in the load of the inverter.